1. Field of the Invention
This invention relates to the manufacture of high quality optical films, and in particular to a method of depositing an optical quality silica film by PECVD. The invention can be applied to the manufacture of photonic devices, for example, Mux/Demux devices for use fiber optic communications.
2. Description of Related Art
The manufacture of integrated optical devices, such as optical Multiplexers (Mux) and Demultiplexers (Dmux) requires the fabrication of optical quality elements, such as waveguides and gratings highly transparent in the 1.30 xcexcm and 1.55 xcexcm optical bands. These silica-based optical elements are basically composed of three layers: buffer, core and cladding. For reasons of simplicity, the buffer and cladding layers are typically of the same composition and refractive index. In order to confine the 1.55 xcexcm (and/or 1.30 xcexcm) wavelength laser beam, the core must have a higher refractive index than the buffer and cladding layers. The required refractive index difference is referred to as the xe2x80x98delta-nxe2x80x99 and is one of the most important characteristics of these silica-based optical elements.
It is very difficult to fabricate transparent silica-based optical elements in the 1.55 xcexcm wavelength (and/or 1.30 wavelength) optical region while maintaining a suitable difference delta-n and preventing stress-induced mechanical and problems. Our co-pending U.S. patent application Ser. No. 09/799,491 filed on Mar. 7, 2000 entitled xe2x80x98Method of Making a Functional Device with Deposited Layers subject to High Temperature Annealxe2x80x9d describes an improved Plasma Enhanced Chemical Vapour Deposition technique for these silica-based elements which allows the attainment of the required xe2x80x98delta-nxe2x80x99 while eliminating the undesirable residual Si:Nxe2x80x94H oscillators (observed as a FTIR peak centered at 3380 cmxe2x88x921 whose 2nd harmonics could cause an optical absorption between 1.445 and 1.515 xcexcm), SiNxe2x80x94H oscillators (centered at 3420 cmxe2x88x921 whose 2nd harmonics could cause an optical absorption between 1.445 and 1.479 xcexcm) and SiOxe2x80x94H oscillators (centered at 3510 cmxe2x88x921 and whose 2nd harmonics could cause an optical absorption between 1.408 and 1.441 xcexcm) after a high temperature thermal treatment in a nitrogen ambient, typically at 800xc2x0 C.
With such a high temperature thermal treatment are associated some residual stress-induced mechanical problems of deep-etched optical elements (mechanical movement of the side-walls) and some residual stress-induced mechanical problems at the buffer/core interface or at the core/cladding interface (micro-structural defects, micro-voiding and separation).
Recently published literature reveals various PECVD approaches to obtain these high performance optically transparent silica-based optical elements: Valette S., New integrated optical multiplexer-demultiplexer realized on silicon substrate, ECIO ""87, 145, 1987; Grand G., Low-loss PECVD silica channel waveguides for optical communications, Electron. Lett., 26 (25), 2135, 1990; Bruno F., Plasma-enhanced chemical vapor deposition of low-loss SiON optical waveguides at 1.5-xcexcm wavelength, Applied Optics, 30 (31), 4560, 1991; Kapser K., Rapid deposition of high-quality silicon-oxinitride waveguides, IEEE Trans. Photonics Tech. Lett., 5 (12), 1991; Lai Q., Simple technologies for fabrication of low-loss silica waveguides, Elec. Lett., 28 (11), 1000, 1992; Lai Q., Formation of optical slab waveguides using thermal oxidation of SiOx, Elec. Lett., 29 (8), 714, 1993; Liu K., Hybrid optoelectronic digitally tunable receiver, SPIE, Vol 2402, 104, 1995; Tu Y., Single-mode SiON/SiO2/Si optical waveguides prepared by plasma-enhanced Chemical vapor deposition, Fiber and integrated optics, 14, 133, 1995; Hoffmann M., Low temperature, nitrogen doped waveguides on silicon with small core dimensions fabricated by PECVD/RIE, ECIO""95, 299, 1995; Bazylenko M., Pure and fluorine-doped silica films deposited in a hollow cathode reactor for integrated optic applications, J. Vac. Sci. Technol. A 14 (2), 336, 1996; Poenar D., Optical properties of thin film silicon-compatible materials, Appl. Opt. 36 (21), 5112, 1997; Hoffmann M., Low-loss fiber-matched low-temperature PECVD waveguides with small-core dimensions for optical communication systems, IEEE Photonics Tech. Lett., 9 (9), 1238, 1997; Pereyra I., High quality low temperature DPECVD silicon dioxide, J. Non-Crystalline Solids, 212, 225, 1997; Kenyon T., A luminescence study of silicon-rich silica and rare-earth doped silicon-rich silica, Fourth Int. Symp. Quantum Confinement Electrochemical Society, 97-11, 304, 1997; Alayo M., Thick SiOxNy and SiO2 films obtained by PECVD technique at low temperatures, Thin Solid Films, 332, 40, 1998; Bulla D., Deposition of thick TEOS PECVD silicon oxide layers for integrated optical waveguide applications, Thin Solid Films, 334, 60, 1998; Valette S., State of the art of integrated optics technology at LETI for achieving passive optical components, J. of Modern Optics, 35 (6), 993, 1988; Ojha S., Simple method of fabricating polarization-insensitive and very low crosstalk AWG grating devices, Electron. Lett., 34 (1), 78, 1998; Johnson C., Thermal annealing of waveguides formed by ion implantation of silica-on-Si, Nuclear Instruments and Methods in Physics Research, B141, 670, 1998; Ridder R., Silicon oxynitride planar waveguiding structures for application in optical communication, IEEE J. of Sel. Top. In Quantum Electron., 4 (6), 930, 1998; Germann R., Silicon-oxynitride layers for optical waveguide applications, 195th meeting of the Electrochemical Society, 99-1, May 1999, Abstract 137, 1999; Worhoff K., Plasma enhanced cyhemical vapor deposition silicon oxynitride optimized for application in integrated optics, Sensors and Actuators, 74, 9, 1999; and Offrein B., Wavelength tunable optical add-after-drop filter with flat passband for WDM networks, IEEE Photonics Tech. Lett., 11 (2), 239, 1999.
A comparison of these various PECVD techniques is summarised in FIG. 1 which shows the approaches and methods used to modify the xe2x80x98delta-nxe2x80x99 between buffer (clad) and core with post-deposition thermal treatment.
The various techniques can be grouped into main categories: PECVD using unknown chemicals, unknown chemical reactions and unknown boron (B) and/or phosphorus (P) chemicals and unknown chemical reactions to adjust the xe2x80x98delta-nxe2x80x99 (When specified, the post-deposition thermal treatments range from 400 to 1000xc2x0 C.); PECVD using TEOS and unknown means of adjusting the xe2x80x98delta-nxe2x80x99 (The post-deposition thermal treatments are not specified); PECVD using oxidation of SiH4 with O2 coupled with silicon ion implantation or adjustment of silicon oxide stoichiometry as means of adjusting the xe2x80x98delta-nxe2x80x99 (The post-deposition thermal treatments range from 400 to 1000xc2x0 C.); PECVD using oxidation of SiH4 with O2 coupled with the incorporation of CF4 (SiH4/O2/CF4 flow ratio) as means of adjusting the xe2x80x98delta-nxe2x80x99 (When specified, the post-deposition thermal treatments range from 100 to 1000xc2x0 C.); PECVD using oxidation of SiH4 with N2O coupled with variations of N2O concentration (SiH4/N2O flow ratio) as means of adjusting the silicon oxide stoechiometry and the xe2x80x98delta-nxe2x80x99 (The post-deposition thermal treatments range from 400 to 1100xc2x0 C.); PECVD using oxidation of SiH4 with N2O coupled with variations of N2O concentration and with the incorporation of Ar (SiH4/N2O/Ar flow ratio) as means of adjusting the silicon oxide stoechiometry and the xe2x80x98delta-nxe2x80x99 (The post-deposition thermal treatments is 1000xc2x0 C.); PECVD using oxidation of SiH4 with N2O coupled with the incorporation of NH3 (SiH4/N2O/NH3 flow ratio) to form silicon oxynitrides with various xe2x80x98delta-nxe2x80x99 (When specified, the post-deposition thermal treatments range from 700 to 1100xc2x0 C.); PECVD using oxidation of SiH4 with N2O coupled with the incorporation of NH3 and Ar (SiH4/N2O/NH3/Ar flow ratio) as to form silicon oxynitrides with various xe2x80x98delta-nxe2x80x99 (The post-deposition thermal treatments are not specified); PECVD using oxidation of SiH4 with N2O coupled with the incorporation of NH3 and N2 chemicals variation (SiH4/N2O/NH3/N2 flow ratio) as to form silicon oxynitrides with various xe2x80x98delta-nxe2x80x99 (The post-deposition thermal treatments range from 850 to 1150xc2x0 C.); PECVD using oxidation of SiH4 with N2O and O2 coupled with the incorporation of CF4, N2 and He (SiH4/(N2O/N2)/O2/CF4 flow ratio) as to form complex mixtures of carbon and fluorine containing silicon oxide as means of adjusting the xe2x80x98delta-nxe2x80x99 (The post-deposition thermal treatments is 425xc2x0 C.).
Our co-pending U.S. patent application Ser. No. 09/833,711 entitled xe2x80x98Optical Quality Silica Filmsxe2x80x99 describes an improved Plasma Enhanced Chemical Vapour Deposition technique for silica films which shows that the independent control of the SiH4, N2O and N2 gases as well as of the total deposition pressure via an automatic control of the pumping speed of the vacuum pump in a five-dimensional space consisting of a first independent variable, the SiH4 flow; a second independent variable, the N2O flow; a third independent variable, the N2 flow; a fourth independent variable; the total deposition pressure (controlled by an automatic adjustment of the pumping speed); and the observed film characteristics; permits the elimination of the undesirable residual Si:Nxe2x80x94H oscillators (observed as a FTIR peak centered at 3380 cmxe2x88x921 whose 2nd harmonics could cause an optical absorption between 1.445 and 1.515 xcexcm), SiNxe2x80x94H oscillators (centered at 3420 cmxe2x88x921 whose 2nd harmonics could cause an optical absorption between 1.445 and 1.479 xcexcm) and SiOxe2x80x94H oscillators (centered at 3510 cmxe2x88x921 and whose 2nd harmonics could cause an optical absorption between 1.408 and 1.441 xcexcm) after thermal treatment at a low post-deposition temperature of 800xc2x0 C. to provide improved silica films with reduced optical absorption in the 1.55 xcexcm wavelength (and/or 1.30 xcexcm wavelength) optical region.
Another co-pending U.S. patent application Ser. No. 09/867,662 entitled xe2x80x98Method of Depositing Optical Filmsxe2x80x9d describes a new improved Plasma Enhanced Chemical Vapour Deposition technique of silica waveguides which shows that the independent control of the SiH4, N2O, N2 and PH3 gases as well as of the total deposition pressure via an automatic control of the pumping speed of the vacuum pump in a six-dimensional space, namely a first independent variable, the SiH4 flow; a second independent variable, the N2O flow; a third independent variable, the N2 flow; a fourth independent variable, the PH3 flow; a fifth independent variable; the total deposition pressure (controlled by an automatic adjustment of the pumping speed); and the observed waveguides characteristics, is key to achieving the required xe2x80x98delta-nxe2x80x99 while still eliminating the undesirable residual Si:Nxe2x80x94H oscillators (observed as a FTIR peak centered at 3380 cmxe2x88x921 whose 2nd harmonics could cause an optical absorption between 1.445 and 1.515 xcexcm), SiNxe2x80x94H oscillators (centered at 3420 cmxe2x88x921 whose 2nd harmonics could cause an optical absorption between 1.445 and 1.479 xcexcm) and SiOxe2x80x94H oscillators (centered at 3510 cmxe2x88x921 and whose 2nd harmonics could cause an optical absorption between 1.408 and 1.441 xcexcm) after thermal treatment at a low post-deposition temperature of 800xc2x0 C. as to provide improved silica waveguides with reduced optical absorption in the 1.55 xcexcm wavelength (and/or 1.30 wavelength) optical region.
While these techniques are capable of producing optical quality films, they can result in stress-induced mechanical problems for deep-etched optical components.
According to the present invention there is provided a method of depositing an optical quality silica film by PECVD (Plasma Enhanced Chemical Vapor Deposition), comprising independently setting a predetermined flow rate for a raw material gas; independently setting a predetermined flow rate for an oxidation gas; independently setting a predetermined flow rate for a carrier gas; independently setting a predetermined total deposition pressure; and applying a post deposition heat treatment to the deposited film at a temperature selected to optimize the mechanical properties without affecting the optical properties of the deposited film.
In a preferred embodiment flow rate for a dopant gas is also independently set. The observed FTIR characteristics of the deposited film are monitored to determine the optimum post deposition heat treatment temperature.
This technique permits the required xe2x80x98delta-nxe2x80x99 to be achieved while eliminating the undesirable residual Si:Nxe2x80x94H oscillators (observed as a FTIR peak centered at 3380 cmxe2x88x921 whose 2nd harmonics could cause an optical absorption between 1.445 and 1.515 xcexcm), SiNxe2x80x94H oscillators (centered at 3420 cmxe2x88x921 whose 2nd harmonics could cause an optical absorption between 1.445 and 1.479 xcexcm) and SiOxe2x80x94H oscillators (centered at 3510 cmxe2x88x921 and whose 2nd harmonics could cause an optical absorption between 1.408 and 1.441 xcexcm) after an optimised thermal treatment in a nitrogen. The technique can provide improved silica-based optical elements with reduced optical absorption in the 1.55 xcexcm wavelength (and/or 1.30 xcexcm wavelength) optical region without the residual stress-induced mechanical problems of deep-etched optical elements (mechanical movement of side-walls), without the residual stress-induced mechanical problems at the buffer/core or core/cladding interfaces (micro-structural defects, micro-voiding and separation) and without the residual stress-induced optical problems (polarisation dependant power loss).